After serving in the British Army during World War I McCoy became a barrister in 1920 and held a number of leading legal positions in Northern Ireland including Crown Prosecutor for County Fermanagh (from 1926), Resident Magistrate for Belfast (1937–1943) and Senior Crown Prosecutor for Belfast (1949–1967).
Initially elected to the Parliament in a by-election on 12 April 1945 (following the death of Rowley Elliott the previous year), McCoy held the seat for the Ulster Unionists until his retirement in 1965.
McCoy's ideas were generally rejected by the Unionist establishment, who were generally happy with the way things were, and he was sidelined, although he did serve as Speaker of the House of Commons of Northern Ireland in 1956 during a brief period when the long-term Speaker Sir Norman Stronge was forced to step aside.
An office Stronge held was found to disqualify him, but he resigned it and a Bill was rushed through Parliament to indemnify him.
McCoy stepped down from the Northern Ireland House of Commons in 1965, when his seat was won by John Taylor.